It comes a day after he told Fairfax Media "we haven't been able to estimate" the numbers of fresh redundancies partly because it was not known how many would overlap with savings measures already in place.

Mr Shepherd's answer to the committee later prompted Labor senator Sam Dastyari to ask: "The 15,000 job figure is in his own report – has he even read it?"

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"It is pretty worrying when the chair of the government’s Commission of Audit can’t even tell you – conclusively – how many jobs are on the line," he said.

"This report will be a blow to the families of Canberra and frankly it is not fair or equitable to place that kind of burden on one city."

The commission's chairman could not be contacted for further comment on Friday, although in the hearing he repeated his point that the public service had a sacred obligation to be efficient and there was "no such thing as government money, there's only taxpayers' money".

His uncertainty about the number of public servants to lose their jobs under his recommendations will give further confidence to the Community and Public Sector Union (CPSU), which said a figure of 25,000 was more appropriate.

Criticism of the report during the committee hearing also came from Labor senator Kate Lundy, who said the commission's recommendation to review the National Capital Authority was shallow and did not reflect the role the organisation played.

The union's figure included 6000 cuts already announced since September and ACT Chief Minister Katy Gallagher referred to those reductions in Friday's Council of Australian Governments meeting.

Ms Gallagher said she told the meeting people needed to care about Canberra as much as they cared about Geelong and other areas in Australia that were losing jobs.

While not all the report's recommendations will be endorsed by Treasurer Joe Hockey's budget in less than a fortnight, Emeritus Professor Meredith Edwards, from the Institute for Governance at the University of Canberra, said any suggestions not brought in would still be considered alive and possibly open to reintroduction in the future.

"I see it as a working document (for the Coalition) going into the future," she said.

It comes as another expert, University of Canberra economist Phil Lewis, said the greatest threat to Canberra's status as the nation's centre of governance would not be mergers, privatisations or job reductions.

While he said they would affect the size of the Australian public service, Mr Lewis said giving more power to the states would account for job cut numbers not shown in many of the headline figures.

He said it would be a radical step given the centralisation that had been occuring in governance recently.

The national curriculum was a prime example of policy being pushed out across Australia from Canberra.

Unions ACT Secretary Kim Sattler said the audit report was useful in one way because other people now saw the Abbott government as a common enemy.

"We'll be working more closely with charities, churches and community sector organisations," Ms Sattler said.

Unions in Canberra will gather for a May Day rally and march on Saturday, starting at 11.30am at Old Parliament House and finishing at Parliament House with a band and a barbecue.

Speakers will include Australian Council of Trade Unions president Ged Kearney, CPSU national secretary Nadine Flood and Ms Lundy.